make inline attachments possible

This makes inline attachments possible.
Which means that npm publish will be at minimum only 1 PUT request,
this will eliminate the problem of getting half published modules on flakey networks,
as well as making publishes faster, with fewer round trips.

I also have an implementation of atomic publishes: https://github.com/dominictarr/npm-atomic-publish

I will turn it into a pull request to `npm-registry-client`,
but it's late now, so I'll leave it until after this is merged.